There are many applications and a great need for resonators exhibiting high Q characteristics, especially in the microwave frequency range. These resonators are often used in filter circuits which provide a deep notch in the response curve or a bandpass response.
In the past, microwave filter circuits generally used high-Q passive waveguide and coaxial resonators which were usually heavy and bulky. This has been a serious limiting factor since many desirable applications of microwave filters require that the size be small and the weight, low.
In order to overcome this problem, attention has been directed at using active filter techniques, in order to reduce the bulk and weight factor. Microwave resonators using bipolar transistors have been designed and constructed, but they have generally not been put to practical use because of instability and noise problems. A reference which may be referred to in this regard is an article by D. K. Adams and R. Y. Ho, entitled "Active Filters for UHF and Microwave Frequencies," IEEE MTT-17, September 1969. It should therefore be evident that a new technique that permits the replacement of high-Q waveguide and coaxial filters by active MIC filters, with reductions of weights and volumes by large factors, would constitute a significant advancement in the art.